Fiber-reinforced composite materials using a thermosetting resin such as an epoxy resin as a matrix resin—in particular, carbon fiber-reinforced composite materials using carbon fibers—are used in a wide range of fields such as the field of structural materials for aircraft, automobiles, and the like, the reinforcement of concrete structures, and sports fields such as golf clubs, tennis rackets, and fishing poles due to the light weight and excellent dynamic properties thereof. Carbon fiber-reinforced composite materials not only have excellent dynamic properties, but carbon fibers also have conductivity, and composite materials thereof have excellent electromagnetic wave-shielding properties. Therefore, carbon fiber-reinforced composite materials are also used for the cases of electronic/electrical equipment such as laptops and video cameras and are useful for reducing the case thickness and reducing the weight of the equipment. Such carbon fiber-reinforced composite materials are often obtained by laminating a prepreg obtained by impregnating reinforcing fibers with thermosetting resin.
Examples of the characteristics required of a prepreg used in such an application include the excellent physical properties of the molded product such as heat resistance and impact resistance, of course, as well as excellent storage stability at room temperature and proper curing under prescribed curing conditions (curing temperature, curing time, and the like).
Of the various applications of fiber-reinforced composite materials, there is a demand for structural materials of aircraft, automobiles, or building materials or the like, in particular, to have flame retardancy so that the materials do not ignite and combust due to fire. There is also a demand for the flame retardancy of materials for electronic/electrical equipment in order to prevent accidents in which a case, a part, or the like ignites and combusts when due to heat generated from within the device or the exposure of the outside to high temperatures.
Halogen flame retardants have been conventionally used to provide flame retardancy to carbon fiber-reinforced composite materials. Examples of halogen flame retardants that have been used are halogenated epoxy resins having a halogen such as bromine in an epoxy resin or flame-retardant epoxy resin compositions which provide flame retardancy to a halogenated epoxy resin by using antimony trioxide (Sb2O3) as a flame retardant. Adding antimony trioxide to a halogenated epoxy resin yields a substantial radical trapping effect in the gaseous phase and air shielding effect in the gaseous phase as well as a high flame retarding effect.
While these halogen flame retardants have excellent flame retardancy when added in small quantities, there is a possibility of generating toxic gases such as halogenated hydrogen and organic halides at the time of combustion, which may have an adverse effect on the human body or the natural environment. In addition, the antimony trioxide used together with halogen flame retardants is harmful and must be handled carefully. Moreover, since antimony trioxide has toxicity with powders, it is preferable for the resin composition to contain no antimony trioxide whatsoever out of consideration of the effects on the human body and the environment. Therefore, flame-proofing with a non-halogen which does not contain halogens or antimony trioxide and demonstrates excellent flame retardancy has been promoted.
During the course of the background described above, epoxy resin compositions formed by combining phosphorus compounds such as red phosphorus and phosphoric acid esters or metal oxides such as magnesium oxide and aluminum oxide have been widely studied as flame retardants to be used instead of halogen flame retardants (see WO/2005/082982). WO/2005/082982 describes an epoxy resin composition for a carbon fiber-reinforced composite material which contains an epoxy resin, an amine curing agent, and a phosphorus compound with a phosphorus atom concentration within a prescribed range so that the material has excellent flame retardancy and dynamic properties, does not generate halogen gas when combusted, and can be suitably used as a fiber composite material.
When a phosphorus compound or a metal oxide is added to a thermosetting resin such as an epoxy resin as an alternative flame retardant, it is necessary for the phosphorus compound or the metal oxide to be added in a large quantity in order to achieve sufficient flame retardancy. However, when the phosphorus compound or the metal oxide is added in a large quantity, there is the problem of the diminishment of the physical properties of a cured product obtained by curing the epoxy resin composition such as a decrease in the strength of the cured product.
In addition, when the curability increases at a low temperature of approximately 120° C., the reactivity typically improves, which leads to the diminishment of storage stability. Therefore, when a conventional epoxy resin composition for a carbon fiber-reinforced composite material is used as a prepreg for an aircraft or the like, the epoxy resin composition must be curable and have excellent storage stability even at a temperature of approximately 120° C.
Therefore, with conventional technology, it has been difficult to obtain a cured product which takes into consideration the effects on the body and the environment and demonstrates excellent physical properties and good flame retardancy. The present situation is thus that no flame-retardant epoxy resin composition which is curable at approximately 120° C., is environmentally sound, demonstrates excellent storage stability, and yields a cured product having good flame retardancy and excellent physical properties has been found as a flame-retardant epoxy resin composition.